Multipath fading is a well-known problem that affects many RF communications systems. Fading occurs when multiple, out-of-phase versions of a narrowband RF transmission arrive at the receive antenna simultaneously, usually as a result of multiple reflections of the transmission. These out-of-phase received signals, when combined, will at least partially cancel each other, thereby reducing the strength or amplitude of the received signal. This problem is especially acute for indoor communications systems because of the large number of reflections that can occur in the indoor environment.
Another effect observed in the indoor propagation environment is shadowing. This is caused by blockage of the RF propagation by a large obstruction, and it results in lowering of the mean received signal power. Shadowing is a longer-lived phenomenon than fading. While a deep fade experienced by a moving transmitter might last for milliseconds, shadowing might last for seconds.
One indoor communications system in which fading and shadowing can occur is a system for wireless monitoring of patients in a hospital or other health care facility. In one known wireless remote patient monitoring system, each monitored patient is coupled to a portable patient monitor. Each patient monitor has a transmitting antenna for sending acquired patient monitoring data to a central receiver. To overcome the fading problem, the receiver uses both extra transmit power, known as a fade margin, and a spatial diversity antenna scheme, which is one of several well-known solutions to the problem of multipath fading. This spatial diversity scheme uses four separate antenna fields, each extending throughout the coverage area. When patient monitoring data is being lost due to a low received signal level on one antenna field, the system can switch to a different field that has a higher received signal level.
In the known wireless patient monitoring system described above, a multiplicity of receivers are provided to enable concurrent remote monitoring of a multiplicity of patients. Each receiver can monitor a single antenna field while receiving and demodulating the incoming signal from a single patient. A patient who is being remotely monitored using such a system may move about the hospital freely. The patient's motion causes variations in signal level through fading, shadowing and changes in the closest line-of-sight distance from the patient's transmitter to the nearest antenna of each of the four antenna fields. These variations in signal level make it advantageous for the receiver to switch between antenna fields as the patient moves, so as to maximize the received signal power. Some method of determining the best antenna field to use is required, and this method must be applied over and over again to deal with the effects of patient mobility.
In the known wireless patient monitoring system described above, the incoming signal is structured as a sequence of packets or frames, and the receiver must wait until it starts receiving packets containing errors to determine when to change antenna fields. This results in lost data due to reception errors, alternate antenna field testing, and antenna switching time. For changes in received signal level caused by shadowing or by movement from a position near an antenna of one antenna field to a position closer to an antenna of a second field, the need to switch can be predicted before bit errors start to occur. This prediction can be made based on measured average received signal power over some period of time. The errors in the known system described above are due to the fact that now such prediction is not attempted. Such prediction is not possible in the known system because incoming monitoring data consumes all the bandwidth of the channel.
It would be advantageous to design a technique that could determine when to switch antenna fields without waiting for packets that contain errors. Such a method would eliminate the loss of data due to antenna field switching.